Lactic Acid: The Fermentation By-Product in Your Yogurt and Sourdough

What is lactic acid?

Lactic acid is a short-chain organic acid with the formula C₃H₆O₃. Its European food additive number is E270. It was first isolated from sour milk in 1780 by Swedish chemist Carl Wilhelm Scheele, but humans had been consuming it in fermented foods for thousands of years before that.

The compound exists in two mirror-image forms: L-lactic acid and D-lactic acid. Human metabolism efficiently processes the L-form. Bacterial fermentation typically produces a mixture of both forms, though Lactobacillus strains used in food production are selected to favor the L-form.

Lactic acid is also produced in the human body — in muscle cells during intense exercise, when oxygen supply cannot keep up with energy demand. This is a completely separate metabolic process from dietary lactic acid and has no connection to the additive on your food label.

How lactic acid in food is made

Bacterial fermentation (the commercial standard)

Virtually all food-grade lactic acid produced today is made by bacterial fermentation. Strains of Lactobacillus — the same genus of bacteria responsible for yogurt, sourdough, sauerkraut, and kimchi — are cultured on a carbohydrate substrate and allowed to ferment. The substrate is almost always a plant-derived sugar: corn glucose (dextrose) or beet sugar are the most common in the United States and Europe respectively.

The bacteria convert the sugar into lactic acid. The acid is then separated, purified, and concentrated into a food-grade liquid. No animal products are involved in this process, which is why commercially produced lactic acid is considered vegan by virtually all vegan certification bodies.

Synthetic production

A smaller fraction of global lactic acid supply is produced synthetically via the hydrolysis of acetaldehyde. This route produces racemic lactic acid (equal parts L- and D-forms). Synthetic lactic acid is used more commonly in industrial applications than in food, but it is permitted in food and is also vegan.

What lactic acid does in food

In processed foods, lactic acid serves three main functions, often simultaneously.

pH regulation

Lactic acid is used to lower and stabilize the pH of food products. Controlling pH is critical for both safety (many spoilage organisms cannot grow below pH 4.5) and texture (cheese curd formation, gel structure in puddings, crumb structure in baked goods). It is a common acidulant in processed and preserved foods.

Preservation

By reducing pH, lactic acid inhibits the growth of pathogenic bacteria including Listeria monocytogenes and Salmonella. It is used as a surface treatment on ready-to-eat meats and as a spray on beef carcasses in processing facilities — an application reviewed and approved by both the FDA and USDA.

Flavor

Lactic acid contributes a mild, clean sourness — the tangy note you recognize in yogurt, sour cream, and sourdough bread. It is softer and less sharp than citric or acetic acid, which is why it is preferred in dairy-style flavors and fermented food analogues.

Regulatory status: what the FDA and EFSA say

The FDA classifies lactic acid as Generally Recognized As Safe (GRAS) under 21 CFR 184.1061. This means it has a long history of safe use and a robust scientific consensus supporting its safety at levels used in food. There is no restriction on the quantity that can be added, provided the amount is consistent with good manufacturing practices.

In Europe, lactic acid carries the designation E270 and is approved across all food categories with no maximum level specified — the standard applied is "quantum satis" (as much as needed for the technological purpose). The EFSA Scientific Panel on Food Additives concluded that no acceptable daily intake (ADI) needs to be established, which is among the most favorable assessments a food additive can receive.

Lactic acid in naturally fermented foods

Before it was an additive on a label, lactic acid was simply what made fermented food taste the way it does. During fermentation, Lactobacillus bacteria metabolize sugars and produce lactic acid as a by-product. This drop in pH is what preserves the food and creates the characteristic sour flavor.

Foods that naturally contain lactic acid produced this way include:

• Yogurt and kefir (produced by Lactobacillus bulgaricus and Streptococcus thermophilus)
• Sourdough bread (wild Lactobacillus from starter cultures)
• Sauerkraut and kimchi (fermented cabbage)
• Miso and tempeh
• Naturally fermented pickles and olives (brine fermentation)
• Aged cheeses including cheddar, parmesan, and Swiss

When you see "lactic acid" on an ingredient list, you are seeing the same compound that naturally occurs in all of the above — just added directly rather than produced in situ by bacteria. For more context on how to interpret this on a label, see our guide to reading food labels.

Is lactic acid vegan? Is it dairy?

This is the most common question about lactic acid, and the confusion is understandable: the name sounds like it comes from milk (lacto = milk in Latin), and it is associated with dairy fermentation. But the commercial additive is a different story.

Food-grade lactic acid produced for use as an additive is made by fermenting plant-derived sugars. The bacteria doing the fermenting do not require dairy — they just need a carbohydrate source. Corn glucose and beet sugar are the dominant substrates. The final product contains no milk proteins, no lactose, and no animal-derived material.

Major vegan certification organizations — including The Vegan Society and PETA — consider lactic acid vegan when produced via plant-based fermentation. People with dairy allergies or lactose intolerance can safely consume food additives containing lactic acid without concern.

For a broader look at how to evaluate additives in our lactic acid additive profile, including brands that use it, see the full additive detail page.

Frequently asked questions